The invention relates to a process for measuring the electrical resistance of a resistive body, possibly a resistive substance confined in a given volume, e.g. a conductive liquid contained in a reservoir, the resistance measurement being carried out without contact, i.e. in a non-destructive and non-intrusive manner. The invention also relates to a device for measuring the electrical resistance of a resistive body for carrying out the process and, by way of example, a device for measuring the quantity of a printing product contained in a reservoir, said printing product being resistive.
The invention also relates to a process for checking the conformity of a conductive liquid product contained in a reservoir. It also relates to a device for performing such a process. By way of example, the invention preferably relates to checking the conformity of a conductive printing product in a printer or other office machine incorporating such a printer.
As indicated above, the invention also relates in its operating principle to the measurement of the electrical resistance of a resistive body, because the information which is worked out by performing the process is directly related to the electrical resistance of an analysed resistive body. However, the invention can serve to measure and display any other magnitude, e.g. a quantity of product contained in a reservoir, if the variation in this magnitude is directly linked to that of the resistance by a known relationship.
The principle of a measurement of resistance without contact is already known in some applications. For example, U.S. Pat. No. 3,967,191 describes a process and a device for measurement of the electrical resistance of an internal film of a fluorescent lamp. The resistance of the film is measured by capacitive coupling. To achieve this, two metallic bands forming the plates of a capacitor are placed on the external wall of the lamp. One of the bands is connected to earth via a resistance, while the other is connected to a variable frequency generator via an inductance connected in series. Therefore, the whole constitutes a classic resonant RLC circuit, the resistance R being that of the internal film of the lamp. The frequency of the generator develops at relatively low frequencies, lower than or equal to 15 kHz. The inductance is selected and adjusted so that the resonance is produced during variation of the frequency in a given range. The amplitude of the resonance oscillations is measured to allow the resistance of the internal film of the lamp to be deduced.
Use of a variable frequency generator to detect the resonance frequency of an RLC circuit is a costly method and is a relatively slow process. In some systems, a measurement of this type must be made very rapidly and automatically without the knowledge of the user of the appliance, possibly to be able to prepare and display a message, i.e. to prevent operation of this appliance.
As for a printing machine one or more reservoirs of ink or pigmented product are to be found in an inkjet printer. In the following text the term xe2x80x9cprinting productxe2x80x9d or even xe2x80x9cinkxe2x80x9d are used to refer to any liquid product which is appropriate for this use, including a colourless product known per se permitting better hold of the pigmented products on the paper. Only one reservoir is necessary if it is a black and white printer; if it is a colour printer, there are several reservoirs or compartments provided in the same cartridge which are filled with inks of different colours. Hence, in a high-quality colour printer, there may be up to seven cartridge reservoirs or compartments respectively enclosing inks of the following colours: black, dark cyan, light cyan, dark magenta, light magenta, dark yellow and light yellow. Therefore, it may be difficult for a person with little experience to easily replace or fill the reservoirs when necessary. While an error with black is rare, the risk of errors being made between two shades of the same colour is much more significant. Reversing light cyan and dark cyan reservoirs for example, can impair printing quality. The same applies for the other primary colours.
Moreover, the risks of errors differ according to the design of the printer. For example, in many printers an ejector head is closely associated with one reservoir. Sometimes, the print head is combined with the reservoir and is replaced at the same time as this, if it is a disposable reservoir.
If the system is based on one or more ink cartridges containing several reservoirs or compartments, as well as corresponding print head or heads, there is little risk of any handling error on the part of the user. However, this type of cartridge is expensive and its disposal may cause pollution. In fact, as soon as a reservoir or compartment is empty, the whole cartridge must be changed, which means that costly and polluting printing products are disposed of instead of being used. If the print head is separate from the reservoir or compartment which feeds it, an ink cartridge enclosing all the printing products allows errors to be avoided. Such a cartridge is more economical, but the pollution risks remain significant, since when a reservoir or compartment is empty, the whole cartridge must be changed.
Therefore, it may be preferred to use independent reservoirs, each reservoir possibly being associated with its own ink ejector head. In this case, an interchangeable reservoir only contains one single pigmented product. When one of these is empty, it is sufficient to replace only that one. However, the risks of error are much more significant.
There are also very significant risks of error when the printer comprises a number of reservoirs, which may be refilled by the user as and when required. In this case, there is the additional risk that a reservoir may be filled with an ink which is not appropriate, not only with respect to its exact colour, but also with respect to it quality, and use of such an ink may cause deterioration in the corresponding ink ejector head.
In general, the process for measuring the electrical resistance in accordance with the invention is distinguished from the prior art in that it uses a resonant circuit comprising a variable capacitor forming means and in that the said resonant circuit is supplied with a fixed frequency. Advantageously, the variable capacitor forming means is a variable capacitance diode associated with an adjustable voltage generator, the advantage of this assembly being that it can be operated easily and quickly by varying the voltage applied to the diode, and the search for resonance and measurement of the corresponding peak amplitude can be operated in a very short time.
More precisely, the invention relates to a process for measuring the electrical resistance of a resistive body consisting of defining with said body a capacitance arrangement formed from a capacitive branch comprising at least one capacitor electrically connected to said resistive body and incorporating this capacitive section in a resonant circuit, characterised in that this resonant circuit additionally comprises a variable capacitor forming means, said resonant circuit being supplied with a fixed frequency, the capacitance of said capacitor being varied by measuring a signal delivered by said resonant circuit, the peak amplitude of this signal being measured and the value of this peak being representative of the resistance of said body.
As indicated above, the variable capacitor forming means preferably comprises a variable capacitance diode and an adjustable voltage generator connected to apply an inverse voltage to the terminals of said diode. The implementation of the process therefore consists simply of varying the capacitance of the capacitor by varying the voltage of the voltage generator according to a predetermined law. During this time, a peak amplitude detector connected to the resonant circuit allows said peak to be identified and its amplitude measured.
If the magnitude which has to be measured and checked is not, strictly speaking, the resistance of the resistive body, the process indicated above is supplemented by establishing a correlation between the values of the amplitude of said peak and predetermined values, written in memory, of a variation function of another variable dependent on said resistance.
For example, this other variable can be the quantity of a printing product contained in a reservoir provided that this printing product constitutes a resistive substance. In this case, at least one metal plate forming the capacitor is coupled to the reservoir of the printing product to form said capacitive branch. The process of measurement is performed and a value representative of the quantity of printing product remaining in the reservoir is deduced from the peak amplitude.
In this example, the capacitive branch can be considered as a series connection between the sought resistance and at least one capacitor formed by said metal plate acting as a plate of the capacitor, the insulating wall of the reservoir acting as dielectric, and the surface of the resistive body with respect to said metal plate. Preferably, the frequency supplying said resonant circuit is chosen in a range of frequencies for which it has been confirmed that the variation in capacitance was relatively independent of the quantity of ink contained in the reservoir. This is also the case in particular when the liquid printing product impregnates a spongy mass filling the reservoir.
The invention also relates to a device for measuring the electrical resistance of a resistive body, comprising a resonant circuit including a capacitive arrangement incorporating said resistive body to form a capacitive branch comprising at least one capacitor electrically connected to said resistive body, characterised in that the said resonant circuit additionally comprises a variable capacitor forming means, a fixed frequency oscillator, control means for variation of said variable capacitor, means for detecting a peak of a signal delivered by said resonant circuit, means of measuring the amplitude of this peak and means for preparing a signal representative of the said resistance of the said body.
By way of application, the invention also relates to a device for measuring the quantity of a printing product contained in a reservoir, said printing product being resistive, comprising a capacitive arrangement including at least one metal plate forming a capacitor plate, said reservoir and said printing product, and defining a capacitive branch, a resonant circuit incorporating the said capacitive branch, a fixed frequency oscillator, a variable capacitor forming means, control means for variation of said variable capacitor, means of detecting a peak of the signal delivered by said resonant circuit, means of measuring the amplitude of said peak and means for preparing a signal representative of a quantity of the printing product contained in said reservoir depending on the measured value of the peak amplitude.
The invention also proposes a process for checking the conformity of the ink used in such a context, and more generally checking the conformity of an electrically conductive liquid product.
In fact, it should be noted that the invention applies when the liquid product or products used have a certain resistivity. The invention consists in establishing a correlation between the resistivity of the product contained or reintroduced into the reservoir and its suitability for use with total safety in the device using such a conductive liquid product.
More precisely, the invention relates to a process for checking the conformity of a conductive liquid product contained in a reservoir, characterised in that it comprises forming a capacitive branch including the reservoir in question, and incorporating this capacitive branch in an oscillating circuit, applying an excitation signal to this oscillating circuit, picking up a resulting signal transmitted by said oscillating circuit, deducing from this resulting signal a value representative of the resistivity of the product contained in said reservoir, comparing this value to a prescribed interval of values, and at least producing a signal if the said value is outside the said interval.
In fact, it is established that in the field of printers using conductive inks of different colours, each ink has a specific resistivity. Therefore, a predetermined interval of resistivity can be attributed to it as a test of conformity. In practice, for a printer comprising several different inks, the intervals in question are not the same. Therefore, measurement of the resistivity of the replacement ink allows determination of whether this product is correct, so that the user can be made aware of the problem.
The system may possibly be supplemented by inhibitor means for the printing device, if a conformity check reveals use of an inadequate product.
Depending on the case, the process could be carried out after each change of reservoir or cartridge.
In order to obtain reliable measurements which may be easily used, the capacitive branch is incorporated in a resonant circuit and the resulting signal is picked up in correlation with the resonance conditions of the said resonant circuit. In other words, a correlation is established between the quality factor of the resonant circuit thus formed and the resistivity of the product inserted, from the electrical viewpoint, in said capacitive branch.
According to a preferred embodiment, the resonant circuit comprises a variable capacitor forming means, i.e. preferably a variable capacitor-type diode, the resonant circuit is supplied by a fixed frequency alternating signal, the capacitance of said variable capacitor is varied while measuring a said resulting signal delivered by said resonant circuit, the amplitude of the peak of this signal is measured and the value of this peak is representative of the resistivity of the said body. This type of measurement can be performed for a given reservoir each time this is replaced or filled. The correlation is effected simply after converting the amplitude of the peak into numerical data, consulting a look-up table recorded in a read-only memory of the control system to deduce therefrom the conformity of the ink contained in or introduced into the corresponding reservoir, via its resistivity.
The invention also relates to a device for checking the conformity of a conductive liquid product contained in a reservoir comprising a capacitive arrangement including the said reservoir to achieve a capacitive branch formed by at least one capacitor which is electrically connected to the conductive liquid product contained in said reservoir, characterised in that it comprises: means for incorporating the said capacitive branch into an oscillating circuit, means for exciting this oscillating circuit, means for picking up a resulting signal transmitted by the said oscillating circuit in response to said excitation signal, means for analysing this signal to deduce therefrom a value representative of the resistivity of the product contained in said reservoir, comparing means for determining whether said value is included in a prescribed interval of values, and means for producing a signal to emit at least one error message if the said value is outside the said corresponding interval.
By way of example, the invention also relates to any office machine comprising a device for checking conformity according to the above definition, in particular a printer or facsimile machine. The invention also relates to any microcomputer comprising at least one printing device fitted with a device for checking conformity according to the above definition.
In all cases, the office machine could comprise means for inhibiting a printing system including at least such a reservoir of printing product. These inhibitor means would be operated by a signal produced if an aforementioned.
The invention will be better understood and will be clearer in the light of the following description of a device for measuring the electrical resistance of a resistive body and of a device for measuring the quantity of printing product contained in a reservoir in an office machine comprising a printer, these two devices performing the process defined above, given only by way of example and with reference to the accompanying drawings, in which: